Commercial alloys provide good resistance to carburization and oxidation to temperatures of the order of 1000.degree. C. (1832.degree. F.). How, where, where higher temperatures are combined with severe mixed oxidant environments under high-load conditions, the availability of affordable alloys meeting all the material requirements becomes virtually nil. The failure of commercial alloys to perform at these elevated temperatures can be traced to solutioning of the strengthening phases. The solutioning of these phases lowvers strength and leads to the loss of performance of the protective scales on the alloy due to such mechanisms as scale spallation, scale vaporization or loss of the ability to inhibit or retard cation or anion diffusion through the scale.
Pyrolysis tubing suitable for producing hydrogen from volatile hydrocarbons must operate for years at temperatures in excess of 1000.degree. C. (1832.degree. F.) under considerable uniaxial and hoop stresses. These pyrolysis tubes must form a protective scale under normal operating conditions and be resistant to spallation during shutdowns. Furthermore, normal pyrolysis operations include the practice of periodically burning out carbon deposits within the tubes in order to maintain thermal efficiency and production volume. The cleaning is most readily accomplished by increasing the oxygen partial pressure of the atmosphere within the tubes to burn out the carbon as carbon dioxide gas and to a lesser extent carbon monoxide gas.
Pyrolysis tubes' carbon deposits however, seldom consist of pure carbon. They usually consist of complex solids containing carbon, hydrogen and varying amounts of nitrogen, oxygen, phosphorus and other elements present in the feedstock. Therefore, the gas phase during burnout is also a complex mixture of these clements, containing various product gases, water vapor, nitrogen and nitrogenous gases. A further factor is that the formation of carbon dioxide gases is strongly exothermic. The xothermicity of this reaction is further enhanced by the hydrogen content of the carbon deposit. Thus, although it is standard practice to control the oxygen partial pressure during carbon burnout in order to prevent runaway temperatures, variations in the character of the carbon deposits can lead to so-called "hot spots", i.e., sites hotter than average and "cold spots" i.e., sites cooler than average. Thus, pyrolysis tube alloys over their lifetime are exposed to a broad spectrum of corrosive constituents over a wide range of temperatures. It is for this reason that an alloy is needed that is immune to degradation and loss of strength under these fluctuating conditions of temperature and corrosive constituents.
Aside from considerations involved in the oxygen partial pressure during carbon burnout, there is a great range of oxygen partial pressures which can be expected in service in such uses as heat treating, coal conversion and combustion, steam hydrocarbon reforming and olefin production. For greatest practical use, an alloy should have carburization resistance not only in atmospheres where the partial pressure of oxygen favors chromia (Cr.sub.2 O.sub.3) formation but also in atmospheres that are reducing to chromia and favor the formation of Cr.sub.7 C.sub.3. In pyrolysis furnaces, for example, where the process is a non-equilibrium one, at one moment the atmosphere might have a log of PO.sub.2 of-19 atmospheres (atm) and at another moment the log of PO.sub.2 might be -23 atm or so. Such variable conditions, given that the log of PO.sub.2 for Cr.sub.7 C.sub.3 --Cr.sub.2 O.sub.3 crossover is about -20 atm at 1000.degree. C. (1832.degree. F.), require an alloy which is universally carburization resistant.
It is an object of this invention to provide an alloy suitable for pyrolysis of hydrocarbon at temperatures in excess of 1000.degree. C.
It is a further object of this invention to provide an alloy resistant to the corrosive gases produced during carbon burnout of pyrolysis tubes.
It is a further object of this invention to provide an alloy at oxygen partial pressures that favor formation of chomia and pressures reducing to chromia.